It is generally desirable to operate an AC electrical system at a high power factor for the efficient transfer of electrical power. Highest power factor is attained when line current and line voltage are proportional.
An electronic load on an AC electrical system, such as an offline switching power supply, may attain operation at high power factor through a variety of measures.
One such measure is the closed loop control of the line current waveform. The closed loop control regulates the line current waveform in accordance with a line current reference waveform, the line current reference waveform being proportional to the line voltage waveform. In addition, it is generally desirable that the line current reference waveform is controllable in amplitude for the control of power level.
In the practice of implementing a closed loop control of the line current waveform, a general simplification of circuitry is attained by processing the rectified versions of the line current waveform and the line voltage waveform. Accordingly, it is generally desired that the line current reference waveform is proportional to the rectified line voltage waveform. For the sake of brevity the mention of line voltage and line current in the following implicitly refers to the mention of their rectified versions, unless stated otherwise.
There is a general need to provide the line current reference waveform at lowest cost and size.
A simple and straightforward solution of the prior art provides the line current reference waveform in two steps, a first step whereby a signal proportional to line voltage is derived by means of a resistive voltage divider, and a second step whereby the signal is controlled in amplitude by means of an analog multiplier circuit.
Drawbacks of the conventional approach are the general expense of a high voltage resistive divider, and the general expense of an analog multiplier.